Group III metal nitride films, such as gallium nitride (GaN) films, have applications in a range of devices from light emitting diodes (LEDs) to ultraviolet detectors to transistor devices.
These films have commonly been produced by techniques including molecular beam epitaxy (MBE) and metal organic chemical vapour deposition (MOCVD). These techniques are not entirely satisfactory with MOCVD, for example, requiring operating temperatures of approximately 1000° C. while with MBE it is difficult to regulate the production of higher energy species due to the use of relatively low film growth pressures.
More recently, remote plasma enhanced chemical vapour deposition (RPECVD) has been employed to produce films of high quality at considerably lower temperatures which reduces equipment costs and allows the use of temperature sensitive preferred substrates for film deposition. RPECVD also typically employs much higher pressures enabling better control of high energy species resulting in less film damage relative to the MBE process.
The use of RPECVD in the production of group III metal nitride films is disclosed in the inventor's earlier patent application WO2006/034540 (Macquarie University) which is entirely incorporated herein by reference.
One problem which must be addressed during film production is that the active nitrogen species from the nitrogen plasma can be quite damaging to group III metal nitride material as it forms. The damage occurs because the active nitrogen species causes etching of the substrate upon collision during growth due to the relatively high kinetic and/or potential energy it possesses.
In the case of damage from species with high energy, these species, largely charged ionic or electron species, are reduced in RPECVD by the physical distance between the plasma source and the sample surface and by the presence of a relatively high density of unexcited gas molecules between the plasma source and sample at typical pressures of between 0.01 Torr and 15 Torr as described in WO2006/034540. However, as further outlined in WO2006/034540, damage can also result from the longer lived neutral species generated in the plasma if these species have substantial potential energy. The potential energy of neutral atomic and molecular species that may participate in film growth are listed by Newman [N. Newman, “Thermochemistry of III-N Semiconductors” in Gallium Nitride I, edited by J. I. Panove and T. D. Moustakas, Vol. 50 of Semiconductors and Semimetals (Academic Press, 1998) pg. 86-89].
The potential energy of such species can only be transformed or lost in discrete quanta of potential energy, so energy in excess of that lost though chemical formation is largely kinetic, which is lost through collisions and heating mechanisms. The collision damage is often seen as sample etching and can negatively affect the resulting film properties.
An apparatus and method was described in WO2006/034540 which allowed this problem to be addressed to a useful extent. However, the present inventors have now identified a particular apparatus and process details, some elements of which may be broadly described in WO2006/034540, which have been shown to result in greatly improved film quality.